package_rpm.bbclass: Remove empty build directory

Fixed:
$ bitbake acl
$ ls tmp/work/core2-64-poky-linux/acl/2.3.2/build/acl-2.3.2-build/

This empty directory is created by rpmbuild, just remove it like others above.

Note, master branch doesn't have this problem after upgrade to rpm 4.20.1, but
it's not easy to find which patch(es) fixed the problem, and the issues haven't
been fixed completely, there is still an empty SRPMS directory, so just remove
the build directory as others above.

If we don't fix this, there might be warnings when a recipe uses:

install -m 0644 ${B}/* ${DEPLOYDIR}/boot.bin-extracted/

install: omitting directory '${B}/xilinx-bootbin-1.0-build'

(From OE-Core rev: c4f95abacdb7376f8f299679f3f0b2dd1dc28c27)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

meta/classes-global/package_rpm.bbclass

@@ -728,6 +728,7 @@ python do_package_rpm () {
 
     # rpm 4 creates various empty directories in _topdir, let's clean them up
     cleanupcmd = "rm -rf %s/BUILDROOT %s/SOURCES %s/SPECS %s/SRPMS" % (workdir, workdir, workdir, workdir)
+    cleanupcmd += " %s/%s-build" % (d.getVar('B'), d.getVar('P'))
 
     # Build the rpm package!
     d.setVar('BUILDSPEC', cmd + "\n" + cleanupcmd + "\n")